Stem cells are unspecialized cells that can self renew indefinitely and also differentiate into more mature cells with specialized functions. Stem cells offer unprecedented opportunities for treatment of debilitating diseases and a new way to explore fundamental questions of biology. Human embryonic stem cells have been shown to develop into multiple tissue types and to exhibit long-term self renewal in culture; however, the use of human embryonic stem cells is controversial, given the diverse views held in society about the moral and legal status of the early embryo. The controversy has prompted scientists to find meaningful post-natal substitutes for embryonic stem cells.
It is now known that cells having at least some of the characteristics of embryonic stem cells are present in after-born individuals, even throughout adulthood. Often called “adult stem cells,” these cells can self renew for extended periods of time, and can give rise to cells with specialized morphology and function. Until recently, it was not believed that adult stem cells found in one tissue type (such as, a neural stem cell) would be capable of generating the specialized cell types of another tissue type (such as, a blood cell). Moreover, it was not thought that an adult stem cell could differentiate into a cell type derived from an embryonic germ layer other than the one from which the stem cell derived.
Although primitive cell subpopulations with the potential to differentiate toward various cell lineages have been isolated from post-natal sources, few of such cells can maintain a broad and multilineage differentiation capacity, resembling the plasticity of embryonic stem cells. Additional multipotent post-natal cells, particularly from human sources, are needed, as are new methods of reproducibly isolating such cells.